dashboard: consolidate NodeView.has_log into build_log_id

Per mara's review on #2896: has_log: bool was fully redundant once
build_log_id: Option<i64> existed alongside it (has_log was always
just build_log_id.is_some()). Dropped has_log, threading the single
Option<i64> field through job_queue::mod.rs, the hivectl NodeView
test-helper literal, and the one remaining frontend consumer
(findLiveBuild's live-log-panel gate, which now checks
build_log_id != null instead of the separate bool).

Also fixed a now-stale doc comment on GET /api/build-log/{node_id}
that claimed the dashboard used on-demand node-id fetches "instead
of an inline build_log_id on the wire" -- no longer true after this
PR put one there for the BUILD L0GS deep-link.

cargo build/clippy/test clean across the three touched crates; nix
fmt clean; frontend build verified (0 has_log references, 3
build_log_id references in the built builds.js bundle).

hive-c0re/src/job_queue/mod.rs

@@ -560,7 +560,6 @@ impl QueueInner {
                 Vec::new()
             };
             let build_log_id = self.node_rt.get(&id).and_then(|r| r.build_log_id);
-            let has_log = build_log_id.is_some();
             // `node.parent` is the structural jobq parent. Top-level nodes
             // have `parent == Some(container)` (direct children of the Dag
             // container); those become `parent: None` on the wire since the
@@ -581,7 +580,6 @@ impl QueueInner {
                 error: node.error.clone(),
                 approval_id,
                 inputs,
-                has_log,
                 build_log_id,
                 parent,
             });